Size effects in nanoindentation of hard and soft surfaces

Abstract

Nanoindentation experiments carried out with atomic force microscopes (AFMs) open the way to understand size-related mechanical effects that are not present at the macro- or micro-scale. Several issues, currently the subject of a wide and open debate, must be carefully considered in order to measure quantities and retrieve trends genuinely associated with the material behaviour. The shape of the nanoindenter (the AFM tip) is crucial for a correct data analysis; we have recently developed a simple geometrical model to properly describe the tip effect in the nanoindentation process. Here, we demonstrate that this model is valid in indentation of both soft and hard, or relatively hard, materials carried out by two distinct, commercially available, AFM probes. Moreover, we implement the model with a data interpretation approach aimed at preventing underestimation of the tip penetration into the material. Experiments on soft polymeric materials (poly(methyl methacrylate) and polystyrene) and hard or relatively hard (Si, Au, Al) materials are reported. The results demonstrate that true hardness data can be attained also in shallow indentations and that the appearance of size effects strongly depends on data interpretation issues. In addition, we report on stiffness data measured on the considered materials during their nanoindentation.

An inverse method for determining the spatially resolved properties of viscoelastic–viscoplastic three-dimensional printed materials

“…The hardness and reduced modulus show similar behaviour. It is noted also that the hardness is indentation load dependent, as discussed in [ 42 ], while the reduced modulus is relatively load independent if the indentation load is at least 50 mN. These results correlate well with the qualitative observation of varying residual imprints, indicating that the printed VeroClear material properties are z -dependent, and, in particular, depend on the number of printed layers of material.…”

High-resolution high-speed dynamic mechanical spectroscopy of cells and other soft materials with the help of atomic force microscopy

“…As was demonstrated 22 44 45 46 , the use of an excessively sharp probe can easily result in overstretching the material which results in non-linear stress-strain response. It is typically manifest itself in a well-known phenomenon that the measured elastic modulus decreases with indentation and reaches its macroscopic value when a substantial indentation depth is reached (see 47 48 49 50 51 , and references therein). The linearity is attained here by using a sufficiently dull probe.…”

Probing near-surface nanoscale mechanical properties of low modulus materials using a quartz crystal resonator atomic force microscope

“…To evaluate their model, we estimated the moduli using their method, and found values about 50% higher than those obtained from the Oliver-Pharr method, which is still much lower than the bulk properties. Alderighi et al further noted that the apparent moduli could be much lower if the actual tip radii are larger than the nominal values provided by the manufacturers, which is typically the case [5]. In our work the indenter or tip radius is comparable to the indentation depth, therefore the differences in results observed is not a simple issue of tip radius.…”

“…One may expect that the geometry of the AFM tip would play a key role in the results. Alderighi et al proposed a model to take into account the tip geometry [5]. To evaluate their model, we estimated the moduli using their method, and found values about 50% higher than those obtained from the Oliver-Pharr method, which is still much lower than the bulk properties.…”